Why should I learn new languages when I'm never going to be fluent in them due to being past the critical period?

The critical period hypothesis is just that - a hypothesis. There are examples of people who have learned a foreign language past the critical period, eg by immersion method, and have managed to gain as high a degree of fluency as those who learned before the end of critical period.
Ok so, as Krashen says: Younger = better in the long run; but that does not mean that older = impossible!

Don't than. Why should I try to convince you? I have no idea what the "critical period" is in reference to. Go luck on knowing what you are going to know -should cut down on the study time. +5

Enjoyment, for a challenge or something new, and it helps you learn more about your own language.

learning new things expands your mental ability, always

Learning a new language opens up yourself to not just a skill but learning an entire culture and way of life for some people. It makes you human to realize that other people exist outside your language but yet they have same concepts and saying or even jokes. It makes life more enjoyable.

Learning a new language helps you with your own language.